head gasket replacement cost?

Not for sure yet if it is my problem but have an oil leak and have a hard time believing it is all from the valve cover leaking. Have that part and the spark plug tube seals and can buy a head gasket, intake gaskets and exhaust gaskets and head bolts of $140.00. Will do the timing belt at the same time and have a mechanic who charges $45.00/ hour labor and has no problem with me supplying the parts. How many hours is this job and would a head gasket just leak oil and not have coolant problems. Temp gauge is reading fine. 4 quarts of oil in 3000 miles and the mess of it is not going to go on much longer before I fix it but want to get a ball park figure on the time. Thanks in advance!